Processes to avoid anodic oxide delamination of anodized high strength aluminum alloys

ABSTRACT

Methods of forming anodic oxide coatings on high strength aluminum alloys are described. Methods involve preventing or reducing the formation of interface-weakening species, such as zinc-sulfur compounds, at an interface between an anodic oxide coating and underlying aluminum alloy substrate during anodizing. In some embodiments, a micro-alloying element is added in very small amounts to an aluminum alloy substrate to prevent enrichment of zinc at the anodic oxide and substrate interface, thereby reducing or preventing formation of the zinc-sulfur interface-weakening species. In some embodiments, a sulfur-scavenging species is added to an aluminum alloy substrate to prevent sulfur from a sulfuric acid anodizing bath from binding with zinc and forming the zinc-sulfur interface-weakening species at the anodic oxide and substrate interface. In some embodiments, a micro-alloying element and a sulfur-scavenging species are added to an aluminum alloy substrate. Resultant anodic oxide coatings have minimal or no discoloration.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 14/474,021, entitled “PROCESS TO MITIGATE SPALLATION OF ANODIC OXIDE COATINGS FROM HIGH STRENGTH SUBSTRATE ALLOYS,” filed on Aug. 29, 2014; U.S. application Ser. No. 14/593,845, entitled “PROCESSES TO REDUCE INTERFACIAL ENRICHMENT OF ALLOYING ELEMENTS UNDER ANODIC OXIDE FILMS AND IMPROVE ANODIZED APPEARANCE OF HEAT TREATABLE ALLOYS,” filed on Jan. 9, 2015; U.S. application Ser. No. 14/678,881, entitled “PROCESS FOR EVALUATION OF DELAMINATION-RESISTANCE OF HARD COATINGS ON METAL SUBSTRATES,” filed on Apr. 3, 2015; and U.S. application Ser. No. 14/678,868, entitled “PROCESS TO MITIGATE GRAIN TEXTURE DIFFERENTIAL GROWTH RATES IN MIRROR-FINISH ANODIZED ALUMINIUM,” filed on Apr. 3, 2015, each of which is incorporated herein in its entirety.

FIELD

This disclosure relates generally to anodic oxide coatings and methods for forming the same. In particular, methods for preventing formation of compounds during anodizing of certain high-strength aluminum alloy substrates that can weaken the interfacial adhesion of a resultant anodic oxide coating are described.

Any publications, patents, and patent applications referred to in the instant specification are herein incorporated by reference in their entireties. To the extent that the publications, patents, or patent applications incorporated by reference contradict the disclosure contained in the instant specification, the instant specification is intended to supersede and/or take precedence over any such contradictory material.

BACKGROUND

Anodizing of aluminum is most commonly performed in sulfuric-acid based solutions, for example, using the processes defined as “Type II” and “Type III” by MIL-A-8625. The resultant anodic oxide coatings provide good wear and corrosion resistance to the substrate, and Type II coatings in particular, have a good cosmetic appearance. On certain alloys, and within certain process constraints, the resulting oxide layer may be clear and substantially colorless, giving a bright metallic appearance which is a highly desirable finish for the aluminum housing of consumer electronic devices. The anodic oxides are also conducive to taking on dyes for coloring. Thus, type II and III anodizing processes are widely used in various industries.

During type II and III anodizing, sulfur-based anions from the sulfuric acid solution become incorporated within the resulting anodic oxide coating. These sulfur-based anions can combine with certain alloying elements originating from aluminum alloy substrates and that accumulate at an interface between the anodic oxide coating and the aluminum alloy substrate. For example, zinc is a common alloying element found in many high-strength aluminum alloys, notably the 7000-series, of which it is the defining alloying element (as per the International Alloy Designation System). Zinc is less readily oxidized than aluminum, and therefore accumulates at the interface between the anodic oxide coating and aluminum alloy substrate. When the sulfur-based anions combine with zinc enriched at the interface, zinc-sulfur compounds form at the interface. It has been found that these zinc-sulfur compounds can weaken adhesion of the anodic oxide coating to the substrate and cause the anodic oxide coating to be susceptible to delamination (i.e., chipping or peeling), particularly in alloys designed to satisfy both a high strength requirement, and anodizing cosmetics.

SUMMARY

This paper describes various embodiments that relate to anodizing processes and anodic oxide coatings using the same. The methods described are used to form anodic oxide coatings with strong interfacial adhesion by avoiding formation of interface-weakening species at an interface between the anodic oxide coatings and underlying substrates during anodizing of aluminum alloy substrates.

According to one embodiment, an enclosure for an electronic device is described. The enclosure includes an aluminum alloy substrate including zinc, magnesium, and a micro-alloying element. A concentration of the micro-alloying element is at most 0.1 weight %. The enclosure also includes an anodic oxide formed on the aluminum alloy substrate. The micro-alloying element is enriched at an interface between the aluminum alloy substrate and the anodic oxide.

According to another embodiment, a method of forming an enclosure for an electronic device is described. The method includes anodizing an aluminum alloy substrate that includes zinc, magnesium, and a micro-alloying element. A concentration of the micro-alloying element within the aluminum alloy substrate is at most 0.1 weight %. The micro-alloying element reduces enrichment of the zinc at an interface between the aluminum alloy substrate and a resultant anodic oxide. Enrichment of the zinc at the interface is associated with reducing an adhesion of the anodic oxide to the aluminum alloy substrate.

According to a further embodiment, a method of forming an enclosure for an electronic device is described. The method includes anodizing an aluminum alloy substrate that includes zinc, magnesium, and a micro-alloying element. A concentration of the micro-alloying element within the aluminum alloy substrate is at most 0.1 weight %. The micro-alloying element reduces the discrepancy between the anodic oxide growth rates on grains having surface orientations of {111} and those of other orientations. Grain structures having {111} orientation associated with preferential anodic oxide growth and defects within the anodized aluminum alloy substrate.

According to an additional embodiment, a part is described. The part includes an aluminum alloy substrate including zinc as an alloying element. The part also includes an anodic oxide coating formed on the aluminum alloy substrate, the anodic oxide coating including a sulfur species incorporated therein, wherein the anodized part is characterized as having a CIELAB b* color space parameter value between −1 and 1.

According to another embodiment, a method of anodizing an aluminum alloy substrate comprising zinc is described. The method includes anodizing the aluminum alloy substrate in a sulfuric acid-based solution. A sulfur species from the sulfuric acid-based solution becomes incorporated within a resultant anodic oxide coating. Some of the zinc becomes enriched at an interface between the anodic oxide coating and aluminum alloy substrate during the anodizing. The aluminum alloy substrate includes a sulfur-scavenging species that binds with the sulfur species preventing at least some of the enriched zinc from forming a zinc-sulfur compound at the interface. The zinc-sulfur compound is to be avoided or minimized because it is reduces the interfacial adhesion between the anodic oxide coating and the aluminum alloy substrate.

According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes an aluminum alloy substrate including zinc and magnesium. The enclosure also includes an anodic oxide coating formed on the aluminum alloy substrate. The anodic oxide coating includes a sulfur species incorporated therein. Some of the sulfur species is bonded with a sulfur-scavenging species that prevents the sulfur species from binding with the zinc. The magnesium may itself act as the sulfur-scavenging species if it is present in a substantial excess over the balanced level required for the formation of zinc-magnesium precipitates to give a certain target strength or hardness in the alloy.

These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements.

FIGS. 1A and 1B show schematic cross-sections of a surface portion of a part, showing how sulfur-based anodic bath anodizing of zinc-containing aluminum alloys can form interface-weakening species.

FIG. 1C shows an EELS graph and a high-resolution microscopic image indicating evidence of interfacial zinc enrichment for an anodized zinc-containing aluminum alloy substrate.

FIG. 2 shows a schematic cross-section of a surface portion of a part formed using a substrate having a micro-alloying element that prevents or reduces formation of interface-weakening species.

FIG. 3 shows a graph indicating interfacial enrichment for a number of elements as a function of Gibbs free energy.

FIG. 4 shows an EELS graph and a high-resolution microscopic image indicating evidence of prevention of interfacial zinc enrichment when using copper as a micro-alloying element.

FIG. 5A shows a graph indicating yellowing effects on anodic films of aluminum alloy substrates with different amounts of copper.

FIG. 5B shows a graph indicating anodic oxide grown uniformity and defect reduction by using copper as a micro-alloying element.

FIG. 6A shows a flowchart illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate using a micro-alloying element.

FIG. 6B shows a flowchart illustrating a process of reducing grain-related defects in an anodized high-strength substrate using a micro-alloying element.

FIG. 7 shows a schematic cross-section of a surface portion of a part formed using a substrate having a sulfur-scavenging species that prevents or reduces formation of interface-weakening species.

FIG. 8 shows an annotated periodic table summarizing some criterion for choosing a suitable sulfur-scavenging species in accordance with some embodiments.

FIG. 9 shows a graph indicating magnesium and zinc concentrations of different commercially available 7000 series aluminum alloys and custom alloy compositions.

FIG. 10 shows a flowchart illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate using a sulfur-scavenging species.

FIG. 11 shows a flowchart illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate using a combination of sulfur-scavenging species and micro-alloying element.

FIG. 12 shows a graph indicating anodic oxide adhesion improvement by using copper as a micro-alloying element and lithium as a sulfur-scavenging species.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, they are intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments.

Described herein are processes for increasing the adhesion strength of anodic oxide coatings on certain high-strength aluminum alloy substrates. Methods involve preventing the formation of interface-weakening species from forming at an interface between an anodic oxide coating and underlying aluminum alloy metal base. An interface-weakening species is an element or compound that resides at this interface and weakens the bond strength between the anodic oxide coating and metal base, thereby rendering the anodic oxide coating susceptible to chipping, peeling, or spalling. A particular type of interface-weakening species is zinc-sulfur species, such as a zinc sulfate or a zinc sulfite. The zinc originates from the aluminum alloy as an alloying element, and the sulfur originates from a sulfur-containing anodizing solution (e.g., sulfuric acid-based solution). A number of other aluminum alloying elements other than zinc-sulfur species have also been shown to form interface-weakening species at the substrate and anodic oxide coating interface.

Methods described herein involve adding one or more elements to the aluminum alloy substrate prior to anodizing so as to prevent or reduce the formation of interface-weakening species at the substrate and anodic oxide coating interface. In some embodiments, the one or more elements enrich at the interface more favorably than the interface-weakening species, which prevents or reduces the enrichment of interface-weakening species at the interface. In some embodiments, the one or more elements bind with sulfur originating from an anodizing solution during anodizing. This prevents or reduces the occurrence of zinc and/or other elements associated with delamination from combining with the sulfur to form interface-weakening species at the interface.

The present paper makes specific reference to aluminum alloys and aluminum oxide coatings, and particularly to 7000-series alloys of aluminum, which comprise zinc-based strengthening precipitates. It should be understood, however, that the methods described herein may be applicable to other types of aluminum alloys—such as 8000-series, which contain lithium and zinc alloying elements—and possibly also to any of a number of other suitable anodizable metal alloys, such as suitable alloys of titanium, zinc, magnesium, niobium, zirconium, hathium, and tantalum, or suitable combinations thereof. As used herein, the terms anodic oxide, anodic oxide coating, anodic film, anodic layer, anodic coating, oxide film, oxide layer, oxide coating, etc. can be used interchangeably and can refer to suitable metal oxide materials, unless otherwise specified.

Methods described herein are well suited for providing cosmetically appealing surface finishes to consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing anodized finishes for housing for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif.

These and other embodiments are discussed below with reference to FIGS. 1A-12. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

FIGS. 1A and 1B show schematic cross-section views of a surface portion of part 100, showing how sulfur-based anodizing (e.g., Type II anodizing) of zinc-containing aluminum alloys can form zinc-sulfur interface-weakening species. Part 100 includes aluminum alloy substrate 102, a portion of which has been converted to anodic oxide 104, which includes anodic pores 110 that are formed during the anodizing process. The region between anodic oxide 104 and substrate 102 can be referred to as interface 108. Substrate 102 includes aluminum matrix 112 and zinc 106, which serves as an alloying element in many aluminum alloy compositions to increase the strength and hardness of the aluminum alloy, with 7000-series aluminum alloys (per The International Alloy Designation System) generally having relatively high levels of zinc 106. In some applications, it is necessary for substrate 102 in a T6 temper to have a yield strength of at least 330 MPa. In some embodiments, this corresponds to a zinc concentration of at least 4 weight %, in some cases as little as 2 weight %. Note that zinc is schematically shown as points 106 in FIGS. 1A and 1B, though it may be either uniformly distributed through aluminum matrix 112, or concentrated within discrete precipitates, or both. Zinc 106 can combine with magnesium (not shown) as another alloying element to form precipitates such as MgZn₂ (the η′ or “eta-prime” phase), which gives substrate 102 its high strength. The atomic % ratio of magnesium to zinc is thus optimally about 1:2. For simplicity, magnesium is not shown in FIGS. 1A and 1B. In addition, most commercially available aluminum alloys contain other alloying elements that are not shown in FIGS. 1A and 1B for simplicity.

During the anodizing process, alloying elements from substrate 102 behave in various ways according to their relative Gibbs free energies for oxide formation. For example, elements that are more readily oxidized than aluminum matrix 112, such as lithium, magnesium, calcium, scandium, yttrium, and lanthanum, are generally anodized along with aluminum matrix 112. They become incorporated into anodic oxide 104 and can then migrate through interface 108 between substrate 102 anodic oxide 104 at rates that depend primarily on the relative mobilities of ions of these elements through anodic oxide 104, which is, in part, determined by the relative strengths of bonding between these ions and oxygen ions within anodic oxide 104. Some ions, including those of lithium, magnesium, and yttrium, migrate through anodic oxide 104 at a faster rate than aluminum ions.

Conversely, alloying elements that are less readily oxidized than aluminum—that is, elements with more positive Gibbs free energies for oxide formation than that of aluminum—tend to become enriched at interface 108. Examples of elements that can become enriched at interface 108 include copper, zinc, nickel, tin, silver, and gold. More discussion with regard to elements and their Gibbs free energies for oxide formation will be provided below.

FIG. 1A shows zinc 106 enriched at interface 108 since zinc 106 has a positive Gibbs free energy for oxide formation compared to that of aluminum. This enrichment of zinc 106 is an event that occurs during the anodizing of substrate 102. The presence of zinc 106 has been found to weaken the adhesion between anodic oxide 104 and substrate 102. That is, the presence of zinc 106 at interface 108 can cause anodic oxide 104 to be susceptible to spalling, chipping, or peeling away from substrate 102. It should be noted that other elements such as nickel and tin (not shown) have also been shown to have similar interfacial weakening effects when positioned at interface 108. In contrast, elements such as copper and gold (not shown) at interface 108 can strengthen the adhesion of anodic oxide 104 to substrate 102.

The enrichment of zinc 106 at interface 108 during the anodizing process cannot generally be avoided by changing parameters of the anodizing process, or by using a typical pre-treatment operation. The enrichment of zinc 106 is a consequence, primarily of the higher Gibbs free energy of oxide formation for zinc 106 compared to the aluminum metal of aluminum matrix 112, and because zinc 106 is less readily oxidized than the aluminum metal of aluminum matrix 112. That is, the aluminum will oxidize in preference over zinc 106 during the anodizing process, resulting in interfacial enrichment of zinc 106 until equilibrium is achieved. It should be noted, that magnesium (not shown) that is a common alloying element used in combination with zinc 106 in high strength 7000-series alloys, has a low Gibbs free energy of oxidation and is preferentially oxidized over aluminum, resulting in no accumulation of magnesium at interface 108.

FIG. 1B shows additional events that occur during the anodizing process. As described above, anodizing processes such as Type II and Type III anodizing processes (as defined by Military Specification Anodizing (MIL-A-8625)) involve the use of a sulfuric acid solution as an electrolyte. It has been shown that sulfur species 114 originating from the sulfuric acid solution become significantly incorporated within anodic oxide 104. Sulfur species 114 can be any sulfur-containing species formed during an anodizing process using a sulfur-containing anodizing solution, such as sulfate and/or sulfide ions. During anodizing, sulfur species 114 move toward substrate 102 and interface 108, as indicated by arrows 115, driven by the applied electric current for anodizing. When sulfur species 114 reaches interface 108, they can combine with enriched zinc 106 to form zinc-sulfur species 116, which can be any zinc and sulfur-containing species such as zinc sulfate and/or zinc sulfide.

Zinc-sulfur species 116 have been found to further weaken interfacial adhesion between anodic oxide 104 and substrate 102, making anodic oxide 104 even more susceptible to spalling, chipping, or peeling. This can be particularly problematic in zinc-rich aluminum alloys, such as some 7000-series alloys, due to the relatively high levels of zinc 106. In this way, both zinc 106 and zinc-sulfur species 116 can be referred to as interface-weakening species. It should be noted that other elements could combine with sulfur species 114 to form sulfates and/or sulfites that can weaken adhesion of anodic oxide 104 to substrate 102. For example, nickel can also form sulfates that detract from adhesion of anodic oxide 104, whereas copper, silver, gold, and various other elements can form oxides in preference over sulfates and generally do not form egregious interfacial sulfates. Thus, the term “interface-weakening species” is not limited to zinc 106 and zinc-sulfur species 116, but can refer generally to species that weakens the adhesion of anodic oxide 104 to substrate 102.

FIG. 1C shows graph 120 and image 122 indicating evidence of interfacial zinc enrichment for an anodized zinc-containing aluminum alloy substrate. Graph 120 represents data collected by electron energy loss spectroscopy (EELS) and image 122 is collected using high-resolution microscopy. In graph 120 and image 122, upper portions correspond to an anodic oxide (Ano) and lower portions correspond to an aluminum alloy substrate (Al), with the dashed line labeled “ano-Al interface” corresponding to an interface region between anodic oxide (Ano) and aluminum alloy substrate (Al). The EELS graph 120 corresponds to a 20 nanometer scan (indicated in image 122) taken across the ano-Al interface. Graph 120 shows lines for oxygen (O) and zinc (Zn), corresponding to relative amounts of oxygen (O) and zinc (Zn) across the ano-Al interface. As shown, oxygen (O) dramatically increases at the ano-Al interface since this corresponds to the transition from metal material (Al) to metal oxide (Ano). Graph 120 also clearly shows that zinc (Zn) accumulates at the ano-Al interface.

As described above, the presence of copper within an aluminum alloy can strengthen the adhesion of an anodic oxide. In fact, many commercially available aluminum-zinc alloys include significant amounts of copper as an alloying element. However, anodizing such commercially available copper-containing alloys results in entraining the copper into and severely discoloring the resultant anodic oxide such that the anodic oxide takes on a distinctly yellow hue. Since the anodic oxide is partially transparent, this can impart a yellow hue to the silvery-colored base metal substrate (e.g., as viewed from surface 101 of part 100 in FIGS. 1A and 1B), which can detract from the cosmetic appeal of the part.

This yellowing can be measured using conventional techniques such as colorimetry using a spectrophotometer, and described according to a color space such as CIE 1976 L*a*b* with a corresponding standard illuminant and white spot such as CIE Standard Illuminant D65. The anodized substrate can be measured while in a non-dyed state—that is, without any color additives such as anodic dyes (e.g., organic or metallic dyes). Note that the D65 (daylight) white spot is used as the reference throughout this document, but F2 (cool white fluorescent) and A (tungsten) will yield similar results, with the colors all falling within approximately 0.1 b* in the region of interest, regardless of which illuminant standard is used.

In general, L*a*b* color space is a model used to characterize colors of an object according to color opponents L* corresponding to an amount of lightness, a* corresponding to amounts of green and magenta, and b* corresponding to amounts of blue and yellow. By convention, higher L* values correspond to greater amounts of lightness and lower L* values correspond to lesser amounts of lightness. Negative b* values indicate a blue color, with more negative b* values indicating a bluer color, and positive b* values indicate a yellow color, with more positive b* values indicating a yellower color. Anodic oxide 104 having b* values greater than 1 will generally have a perceptibly yellow hue. The presence of too much copper or other certain types of alloying elements within substrate 102 and cause part 100 to have b* values greater than 1 when anodic oxide 104 is more than five micrometers in thickness.

Methods described herein can be used to strengthen the bond between anodic oxide and underlying aluminum alloy substrate without substantially yellowing the anodic oxide. A first strategy involves using a class of elements that become enriched at the interface more favorably than zinc and/or other alloying elements associated with delamination. Preventing or reducing the enrichment of these elements at the interface during the anodizing process eliminates or reduces the formation of interface-weakening species at the interface. This first strategy is referred to below as an interface-weakening species enrichment prevention strategy.

A second strategy involves using a class of elements, referred to as sulfur-scavenging elements, which can bind with the sulfur species originating from an anodizing solution during anodizing. This prevents or reduces the occurrence of zinc and/or other elements associated with delamination from combining with the sulfur species to form the interface-weakening species (e.g., zinc-sulfur species 116) at the interface. This second strategy is referred to below as a sulfur-scavenging strategy. In some embodiments, a combination of enrichment prevention and sulfur-scavenging strategies are used. These and other embodiments are described below.

Interface-Weakening Species Enrichment Prevention

One way of increasing the bond strength between an anodic oxide and high-strength aluminum alloy substrate is by preventing or reducing the enrichment of zinc at the interface between the anodic oxide and substrate that would otherwise occur during anodizing. Since zinc can act as an interface-weakening agent, preventing or reducing accumulation of zinc at the interface can increase the adhesion of the anodic oxide to the substrate. Furthermore, if interfacial zinc accumulation is avoided or reduced, this also prevents or reduces the formation of zinc-sulfur compounds at the interface, which is also an interface-weakening agent.

Preventing or reducing zinc enrichment can be accomplished by adding one or more additional elements to the substrate that will enrich at the interface in preference to zinc. To illustrate, FIG. 2 shows a cross-section view of part 200 formed using such an enrichment prevention strategy. Part 200 includes aluminum alloy substrate 202 with anodic oxide 204 formed from a sulfur-containing bath (e.g., sulfuric acid-based bath) anodizing process. Anodic oxide 204 includes pores 210 formed during the anodizing process. Substrate 202 includes alloying element zinc 206, which is incorporated within aluminum matrix 212. Substrate 202 can also include magnesium (not shown for simplicity) that can combine with zinc 206 form MgZn₂ precipitates within substrate 202 to give substrate 202 high tensile strength, as described above. Sulfur species 214 originating from the sulfur-containing anodizing solution becomes incorporated within anodic oxide 204 during the anodizing process.

In addition to zinc 206 (and optionally magnesium), aluminum alloy substrate 202 includes micro-alloying element 216, which is an element that enriches at interface 208 more favorably than zinc 206, and consequently reduces or eliminates enrichment of zinc 206 at interface 208. Micro-alloying element 216 is added in very small concentrations, i.e., less than 0.1 weight %, and in some embodiments preferably in a concentration of 0.02 weight % to 0.05 weight %. These low concentrations have been found to be sufficient to inhibit zinc 206 enrichment at interface 208, without significantly yellowing anodic oxide 204 or negatively altering other alloy properties of substrate 202 such as strength, elongation, electrical or thermal conductivity, and/or corrosion resistance.

The types of micro-alloying element 216 that preferably enrich at interface 208 compared to zinc 206 can be identified based on how easily micro-alloying element 216 oxidizes compared to zinc 206. That is, since the interfacial enrichment of micro-alloying element 216 during anodizing is primarily a consequence of higher Gibbs free energies of oxide formation compared to that of aluminum (of aluminum matrix 212), it may be assumed, to first approximation, that elements with higher Gibbs free energies for oxide formation than zinc 206 will, in turn be preferentially enriched at interface 208 over zinc 206. This approximately limits elements of interest as possible candidates for micro-alloying element 216 to vanadium, phosphorus, tin, tungsten, iron, germanium, cadmium, molybdenum, nickel, cobalt, phosphorus, antimony, bismuth, arsenic, indium, tellurium, copper, thallium, osmium, selenium, iridium, mercury, platinum, silver, and gold (ranked in approximate order of increasing Gibbs free energy for oxide formation, and consequent enrichment relative to zinc 206).

FIG. 3 shows graph 300 indicating interfacial enrichment of a number of elements as a function of Gibbs free energy (ΔG⁰). Graph 300 is a modified version of data provided in Corrosion Science, Vol. 39, No. 4, pp. 731-737 (1997). The x-axis of graph 300 indicates Gibbs free energy (ΔG⁰) for oxide formation of each element. The y-axis of graph 300 indicates an amount of enrichment of each element at the interface between an anodic film and aluminum alloy substrate, expressed in atoms (×10¹⁵) per cm². Graph 300 indicates that vanadium (V), tin (Sn), nickel (Ni), molybdenum (Mo), bismuth (Bi), antimony (Sb), indium (In), copper (Cu), mercury (Hg), silver (Hg), and gold (Au) have higher ΔG⁰ for oxide formation than zinc (Zn), and that these elements also enrich at the interface. This indicates that using higher ΔG⁰ for oxide formation compared to that of zinc is a good first approximation for determining types of micro-alloying elements that can accumulate at the anodic oxide-substrate interface.

FIG. 4 shows graph 400 and image 402 indicating evidence of prevention of interfacial zinc enrichment when using copper (Cu) as a micro-alloying element. Graph 400 represents data collected by electron energy loss spectroscopy (EELS) and image 402 is collected using high-resolution microscopy. In graph 400 and image 402, upper portions correspond to an anodic oxide (Ano) and lower portions correspond to an aluminum alloy substrate (Al), with the dashed line labeled “ano-Al interface” corresponding to an interface region between anodic oxide (Ano) and aluminum alloy substrate (Al). The EELS graph 400 corresponds to a 20 nanometer scan (indicated in image 402) taken across the ano-Al interface. Graph 400 shows lines for oxygen (O), zinc (Zn), and copper (Cu) corresponding to relative amounts of oxygen (O), zinc (Zn), and copper (Cu) across the ano-Al interface. As shown, copper (Cu) accumulates at the ano-Al interface while zinc (Zn) does not. This EELS scan confirms that copper (Cu) micro-alloying element preferentially enriches at the ano-Al interface over zinc (Zn), and can result no zinc (Zn) enrichment at the ano-Al interface.

Some elements may be eliminated as candidates for a micro-alloying element for various reasons. For example, phosphorus is known to weaken the interface between an anodic oxide and substrate, and can therefore be avoided. Lead, mercury, cadmium, thallium, and arsenic may be avoided due to their toxicity, whilst, nickel may be undesirable for applications where skin contact is anticipated. Mercury, bismuth, lead, tin, cadmium, and indium may not constitute practical alloying elements for aluminum due to their low melting points or phase changes occurring within a typical aluminum alloy's thermal processing window.

Assuming that the micro-alloying element is uniformly distributed within aluminum alloy substrate (and preferably within the aluminum matrix) rather than in discrete second phase particles (which can themselves be a cause of cosmetic defect in anodizing), solubility in the aluminum matrix may also be a selection criterion. Iron, for example, can form Al₁₃Fe₄ precipitates, which act as a grain refiner, limiting grain growth during thermo-mechanical processing of the alloy. This may in turn be perceived as a cosmetic defect in the anodic oxide. Assuming that a solubility of about 0.05 weight % or more in the aluminum matrix is a further condition can eliminate platinum, palladium, selenium, tellurium, arsenic, antimony, nickel, cobalt, molybdenum, and tungsten—although some of these elements will be considered and explored in some cases.

Elements such as tungsten, germanium, tellurium, osmium, selenium, iridium, rhodium, platinum, palladium, silver, and gold may be less desirable candidates due to their scarcity or cost. However, since they may only be needed in low concentrations, they may be considered in some cases. Of the remaining elements having higher Gibbs free energies for oxide formation than zinc identified above, including the rare or expensive metals, some, such as copper and gold, enhance precipitation strengthening, and can therefore possibly be used in combination with lower amounts of zinc.

A major consideration for applications that are to be used for cosmetic surface finishes of consumer products is the intrinsic color of the surface of the part, including the anodic oxide, after anodizing. Elements such as iron, copper, and silver can discolor the anodic oxide. As described above, copper, in particular, results in adding a yellow or bronze color to the anodic oxide. This yellow discoloration is noticeable even when copper is added in quantities as low as about 0.1 weight % to about 0.2 weight %, with b* values of greater than 3 when the anodic oxide has a thickness of 10 micrometers or more using processing conditions of a typical Type II anodizing process. Typical zinc-magnesium-copper aluminum alloys such as commercially available aluminum alloy 7010 (with 1.5-2.0 weight % copper) and 7075 (with 1.2-2.0 weight % copper) have severely discolored anodic oxide (b*>>1). This makes anodized 7010 and 7075 aluminum alloys unsuitable for use in certain products, where a silvery-colored aluminum appearance is desired.

Other commercially available zinc and magnesium alloying element-based 7000-series alloys specify maximum levels of copper: notably 0.2 weight % and 0.1 weight % for 7003 and 7005 respectively. But such permitted levels would still be too high for a desired degree of color control (i.e., b*<1), especially as other elements such as manganese are similarly tolerated or specified as 0.3 weight % max and 0.2-0.7 weight % in 7003 and 7005, respectively. The anodic oxide film thickness on such alloys could be restricted to just a couple of micrometers to minimize discoloration, but that approach severely limits the process window for anodizing parts, and consequently limits the wear and corrosion protection offered by the anodic oxide.

To achieve a desirable level of high clarity (e.g., L*>80, and preferably L*>85) and substantially colorlessness (e.g., b*<1, and preferably b*<0.5) of an anodic oxide, formed under typical Type II anodizing conditions to thicknesses of 10 micrometers or more, the aluminum alloy composition specification for a high-strength 7000-series alloy must, for example, specify that with the exception of zinc and certain corresponding precipitate-forming strengthening element or elements (e.g., magnesium or lithium). For example, aluminum alloy can have strict limits on all elements that would result in discoloration of the anodic oxide or cause other cosmetic defects. For example, limits might be set at 0.01 weight % maximum for chromium, copper, manganese and zirconium, 0.02 weight % maximum for titanium, 0.05 weight % maximum for silicon, 0.08 weight % maximum for iron and 0.01 weight % maximum for any other non-specified element, to a total maximum concentration of 0.1 weight % of other non-specified elements. Note that this range of elemental composition is provided by way of example for yielding substantially colorless anodic oxides, and are not intended to limit the possibility of other variations that would fall within the scope of inventive embodiments presented herein. That is, the concentrations of chromium, copper, manganese, zirconium, titanium, silicon, iron, and/or other non-specified elements can be slightly varied from those listed above and still achieve anodic oxides with acceptable levels of clarity.

Aluminum alloy substrate compositions without copper can offer maximum clarity and colorlessness of the anodic oxide. However, the absence of copper in these alloys can result in more egregious accumulation of zinc at the interface, and necessitates the development of the alternative strategies for delamination mitigation. In the present work, the micro-alloying element is added in controlled “micro-alloying” amounts (<0.1 weight %) to substrates made of high strength aluminum alloys, such as 7000-series alloys, for the specific purpose of eliminating interfacial enrichment of zinc and/or other delamination species. The micro-alloying element is added in specified levels just sufficient to inhibit enrichment of zinc and/or other delamination species at the interface without significant discoloration of the anodic oxide and the resulting surface finish of the part (i.e., unlike the commercially available 7003, 7005, and 7010 alloys), and without significantly altering other alloy properties of the substrate such as strength, elongation, electrical or thermal conductivity, or corrosion resistance.

The amount of micro-alloying element can depend on the type of micro-alloying element and on cosmetic requirements. For example, as described above, even relatively small amounts of copper within aluminum alloy substrates have been found to result in discolored anodized part. To illustrate, FIG. 5A shows graph 500 indicating yellowing effects of different amounts of copper as a micro-alloying element to an aluminum alloy substrate. Graph 500 shows b* color space values for zinc-magnesium aluminum alloy substrates having different amounts of copper anodized using different anodizing bath temperatures. Data for three type of zinc-magnesium aluminum alloy substrates having different concentrations of copper: 1A=0.05 weight % copper; 1B=0.1 weight % copper; and 1C=0.15 weight % copper. In addition to the copper, each type of aluminum alloy substrate 1A, 1B, and 1C include only magnesium and zinc as alloying elements.

Each type of substrate 1A, 1B, and 1C was anodized using 1.5 A/dm² current density in 200 g/L sulfuric acid solution, to a target thickness of 15 micrometers. Three different anodizing bath temperatures were explored: 17 degrees Celsius, 20 degrees Celsius, 23 degrees Celsius, and 26 degrees Celsius. Graph 500 indicates a linear relationship between levels of copper and b* value, with higher concentrations of copper correlating with higher b* values. As described above, higher b* values correspond to yellower appearance—thus the more copper added, the yellower the appearance of the anodized part. Graph 500 also indicates that increasing anodizing bath temperatures can reduce values b*. However, higher anodizing temperatures can result in a softer the anodic oxide. Thus, higher anodizing temperatures may not be suitable for certain applications. For good wear protection, a temperature of 20 degrees Celsius is generally preferred, together with a thickness of 10 micrometers or more. Graph 500 indicates that in embodiments where zinc-magnesium aluminum alloy substrates are required to have b* values no more than 1, the copper concentration should not exceed about 0.1 weight %, depending, in part, on the temperature of the anodizing bath. Similarly, lower current density may be used to reduce the yellowing, but the effect is not as strong as that of raising temperature, and reduced current density again softens the resulting anodic oxide, with 1.5 A/dm² being preferred for sufficient hardness at room temperature.

An additional consideration in determining how much micro-alloying element should be used is the commercial recyclability of the part. By restricting the level of the micro-alloying element to 0.05 weight % or less, there is no implication for recycling of the part. This is because most commercial 7000-series alloys specify a maximum of 0.05 weight % for “other” alloying elements and could therefore accommodate the part having micro-alloying element at levels of 0.05 weight % or less. Thus, in some embodiments, the micro-alloying element is preferably added in amounts less than about 0.05 weight %. This is particularly true of less common alloying element candidates (such as silver, antimony), and is generally not a limiting factor for copper, which is used in most 7000-series alloys at levels of at least 0.5 weight %.

In one preferred embodiment, a specified micro-alloying addition of between 0.02 weight % and 0.05 weight % copper is made to an aluminum alloy that includes about 5.5 weight % zinc and about 1 weight % magnesium, with substantially no other alloying elements. This composition corresponds to a relatively pure and balanced aluminum-zinc-magnesium 7000-series alloy optimized for a high yield strength (about 340-350 MPa), hardness (about 125 HV), and heat-treatability, with a very small amount of copper. It has been found that anodizing this substrate composition using Type II anodizing (200 g/L sulfuric acid bath, at 20 degrees Celsius, with 1 A/dm² current density) to form an anodic oxide having a thickness of about 15 micrometers completely eliminates delamination of the anodic oxide, as assessed by rock tumble testing or by indentation testing such that described in U.S. application Ser. No. 14/678,881, which is incorporated by reference herein in its entirety. The color of the anodized surface of this substrate composition remains silvery, with a b* value of about 0.4, corresponding to very little yellowing.

Similar results were obtained with silver as the micro-alloying element, although for a 0.05 weight % addition, the corresponding atomic % of silver is lower than copper, and the delamination resistance was not as improved as found with copper. In some embodiments, approximately 0.1 weight % silver micro-alloying element is required to eliminate delamination in a 15 micrometer thick anodic oxide, and at that level, the discoloration is found to be higher than that using copper, albeit b* is still less than 1.

In some embodiments, the micro-alloying element includes a combination of copper and silver so as to give compounded benefits in terms of delamination resistance, but also compounded color shifts. One or more of germanium, osmium, iridium, rhodium, and gold may be used similarly to silver and copper, in some cases even without any corresponding yellow discoloration. However the cost premium over silver may make these elements less desirable.

Vanadium was also evaluated, even though its Gibbs free energy for oxide formation is very close to that of zinc. A very slight improvement in delamination resistance was observed, when using 0.05 weight % vanadium concentration, and there was no yellow significant discoloration. Titanium, and zirconium were also evaluated. However, in one embodiment, titanium and zirconium showed no ability to reduce interfacial enrichment of zinc and no significant improvement in delamination resistance, even at 0.3 weight %. This is consistent with the Gibbs free energies for oxide formation for each of titanium and zirconium being lower than that of zinc.

Of the elements as candidates for the micro-alloying element that might be dismissed on the basis of limited solubility in the aluminum matrix, nickel, molybdenum, and antimony were explored at 0.05 weight %. Nickel was detrimental—perhaps forming even worse interfacial compounds than zinc. Molybdenum was of no significant benefit—possibly because its Gibbs free energy for oxide formation is not far enough from that of zinc, compounded by the fact that its relatively high atomic weight makes its corresponding atomic concentration low. Antimony was of significant benefit—comparable to silver, but without the undesirable yellow discoloration of silver. Instead, antimony gave a slight blue discoloration, with a b* value of −0.2. This could possibly be used in combination with a yellowing element to neutralize color. However, antimony can introduce small spherical inclusions to the anodic oxide—probably corresponding to aluminum-antimony precipitates in the alloy, which inhibit growth of a completely uniform anodic oxide film. However, it is possible for more than one type of micro-alloying element to be used to achieve a cumulative effect in offsetting interfacial enrichment of zinc and/or discoloration of the anodic oxide. For example, an element that results in adding a yellow hue (b*>0) to the anodic oxide, such as copper, can be added to increase interfacial enrichment of zinc 206, while an element that results in adding a blue hue (b*<0) to the anodic oxide, such as antimony, can be added to offset the yellowing of the copper. In some embodiments, a target b* for the part is between −1 and 1. In some embodiments, a target b* for the part is between −0.5 and 0.5.

Given the above-described considerations and limitation, in some embodiments, preferred candidates for the micro-alloying element include one or more of vanadium, germanium, cobalt, antimony, copper, tellurium, osmium, selenium, iridium, rhodium, palladium, silver, and gold. In some embodiments, preferred candidates for the micro-alloying element include one or more of copper, silver, and antimony. In some embodiments, more than one element is used in combination. In some embodiments, the micro-alloying element is added at levels of no more than about 0.1 weight %. In some embodiments, the micro-alloying element is preferably added at levels of between 0.02 weight % and 0.05 weight %. In some embodiments, the anodized part, as viewed from an exposed surface of the anodic oxide (surface 201), has a b* value of less than 2, in some embodiments a b* value less than 1, even when anodized to thicknesses of 10 micrometers or more, at current densities of 1 A/dm² or more and an anodizing bath temperature of 25 degrees Celsius or less. In some embodiments, the anodized part has an L* value (corresponding to a level of brightness) greater than 75, and in some preferred embodiments, the anodized part has an L* value greater than 85.

In one preferred embodiment, the specified addition of copper as a micro-alloying element at a level of between 0.02 and 0.05 weight % is notably within the typical tolerances for copper as an impurity in certain commercially available 7000-series alloys, such as 7003 and 7005, which respectively specify 0.1 and 0.2 weight % maximum for copper. A crucial distinction between such tolerated impurity levels in commercially available 7000-series alloys and the specified micro-alloying additions made in embodiments presented herein, however, is the specified micro-alloying addition. In particular, in embodiments presented herein, copper has both a specified minimum (e.g., 0.02 weight %) to ensure sufficient interfacial adhesion of the anodic oxide, and a specified maximum (e.g., 0.05 weight %) that is carefully selected to limit the maximum discoloration of the anodic oxide, and as such is also generally lower than any commercial high strength 7000-series alloys' tolerated impurity level for copper.

There can be other advantages of using micro-alloying element within the aluminum alloy substrates. For example, one problem encountered in the anodizing of aluminum alloys only having zinc and magnesium as alloying elements is the differential growth of an anodic oxide on grains of different surface orientation within the substrate, resulting in grain texture-related thickness variation. In the presence of zinc, grains of {111} surface orientation are relatively anodic, as compared to grains of {110} and {100} orientation, and are thus anodized anomalously fast. This can detract from the aesthetics of the anodized finish of a part—notably as apparent pits at the anodic oxide and substrate interface.

One solution to the problem of differential growth rates for grains of different surface orientation is described in U.S. application Ser. No. 14/678,868, which is incorporated by reference herein in its entirety. In the U.S. application Ser. No. 14/678,868, an electrolyte to enable anodizing at low current density and/or increased temperature whilst maintaining adequate anodic oxide hardness is described. In the present application, the micro-alloying element can induce faster growth of the anodic oxide in other orientations (e.g., {110} and/or {100}) to mitigate the discrepancy. For example, copper micro-alloying element has shown to induce faster anodic oxide grown on grains of {110} orientation. In one embodiment, 0.05 weight % copper proved sufficient to dilute the cosmetic defect observed when Type II anodizing (200 g/L sulfuric acid with 1.5 A/dm² current density) beyond the limit of perception—even on a substrate with a mirror-lapped surface anodized to a thickness of 10 micrometers or more. This corresponds to a thickness discrepancy of less than 5% between grains of distinct orientation—far less than the typical 10% thickness discrepancy that would result in the absence of the micro-alloying addition. In this way, the presence of a micro-alloying element can also reduce non-uniform growth of the anodic oxide and related cosmetic defects.

FIG. 5B shows graph 510 indicating the effect of adding 0.05 weight % copper on a thickness uniformity of a resultant anodic oxide. Graph 510 shows that the absence of the copper addition (5.5 Zn, 1.0 Mg sample), the presence of zinc in the alloy results in very non-uniform growth rates for different crystallographic orientations, with surfaces of orientations close to {111} orientation in particular growing at an accelerated rate. This results in grains of {111} orientation having an oxide film of about 3-9% thicker than the average film thickness—appearing as distinct “pit”-like features in the anodic oxide. These are particularly evident on a substrate that has been lapped to a mirror finish. The addition of 0.05 weight % copper (5.5 Zn, 1.0 Mg, 0.05 Cu sample) is sufficient to overcome preferential growth of oxide on {111} surfaces and to ensure film thickness uniformity within 5%, without having to resort to such methods as that disclosed in U.S. patent application Ser. No. 14/678,868, which is incorporated herein in its entirety.

FIG. 6A shows flowchart 600, illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate. At 602, a micro-alloying element is incorporated into an aluminum alloy substrate. The aluminum alloy substrate can include zinc and magnesium alloys that give the substrate high tensile strength. In some embodiments, the substrate is an enclosure, or part of enclosure, for an electronic device. The micro-alloying element is added in an amount that is less than conventionally used for alloying purposes. In some embodiments, a concentration of the micro-alloying element within the aluminum alloy substrate is at most 0.1 weight %, and in some embodiments about 0.05 weight % or less. The micro-alloying element can be characterized as having a higher Gibbs free energy of oxide formation than the zinc. In some embodiments, the micro-alloying element includes one or more of vanadium, tin, nickel, molybdenum, germanium, bismuth, cobalt, antimony, tellurium, osmium, selenium, indium, iridium, rhodium, palladium, copper, mercury, silver, and gold. In some embodiments, the micro-alloying element includes one or more of copper, silver, and antimony. In particular embodiments, two or more types of micro-alloying elements are used, such as copper and silver.

At 604, the aluminum alloy substrate is anodized. In some embodiments, the anodizing takes place in an anodizing solution comprising sulfuric acid. In some embodiments, a type II anodizing process is used. During anodizing, the micro-alloying element enriches at an interface between the substrate and a resultant anodic oxide, thereby preventing or reducing enrichment of zinc at the interface. Since zinc can be an interface-weakening species, prevention or reduction of zinc accumulation at the interface can increase an adhesion strength of the anodic oxide to the substrate. Furthermore, this prevents or reduces formation of zinc-sulfur species, another interface-weakening species, at the interface.

FIG. 6B shows flowchart 610 illustrating a process of reducing grain-related defects in an anodized high-strength substrate. The aluminum alloy substrate can include zinc and magnesium alloys that give the substrate high tensile strength. At 612, a micro-alloying element having a higher Gibbs free energy of oxide formation than the zinc is incorporated into an aluminum alloy substrate. A concentration of the micro-alloying element within the aluminum alloy substrate can be at most 0.1 weight %, and in some embodiments about 0.05 weight % or less. In some embodiments, the micro-alloying element includes one or more of vanadium, tin, nickel, molybdenum, germanium, bismuth, cobalt, antimony, tellurium, osmium, selenium, indium, iridium, rhodium, palladium, copper, mercury, silver, and gold. In some embodiments, the micro-alloying element includes one or more of copper, silver, and antimony. In particular embodiments, two or more types of micro-alloying elements are used, such as copper and silver.

At 614, the aluminum alloy substrate is anodized, using for example, a sulfuric acid anodizing solution. In some embodiments, a type II anodizing process is used. During anodizing—even at 1.5 A/dm² in a sulfuric acid solution, the presence of the micro-alloying element reduces the discrepancy between the growth rates of anodic oxide on grains of {111} orientation and other orientations. This may be because the micro-alloying element increases the growth rate of the anodic oxide on surface orientations other than {111}—such as {110} and {100} grain orientations. It may also depress the anomalously high growth rate of {111} oriented grains. The result is an oxide with thickness uniformity to within 2-3% between grains of distinct orientations—far less than the 10% discrepancy that would result in the absence of the micro-alloying addition. The resultant anodized substrate is free from pitting defects related to accelerated anodic oxide grown at {111} grains. This can be especially important in anodized substrates that have an underlying polished and highly uniform surface (e.g., having a mirror shine).

Sulfur-Scavenging

Another way of increasing the bond strength between an anodic oxide and high-strength aluminum alloy substrate is by preventing or reducing the occurrence of sulfur bonding with zinc during anodizing, thereby preventing or reducing formation of zinc-sulfur species at the interface. As described above, zinc-sulfur species can act as an interface-weakening species—thus, eliminating or minimizing the formation of such zinc-sulfur can increase the adhesion strength of the anodic oxide.

This can be accomplished by adding different class of alloying elements to aluminum alloy substrate that will preferentially bond with sulfur, thereby preventing the sulfur from bonding with zinc. To illustrate, FIG. 7 shows a cross-section view of part 700 formed using such a sulfur-scavenging strategy. Part 700 includes aluminum alloy substrate 702 with anodic oxide 704 formed from a sulfur-containing bath (e.g., sulfuric acid-based bath) anodizing process. Pores 710 of anodic oxide 704 are formed during the anodizing process. Substrate 702 includes alloying element zinc 706 and optionally magnesium (not shown for simplicity) that can combine with zinc 706 form precipitates such as MgZn₂ and give substrate 702 high tensile strength, as described above. Sulfur species 714 originating from the sulfur-containing anodizing solution becomes incorporated within anodic oxide 704 during the anodizing process. Sulfur species 714 is likely in ionic form, such as a sulfide, and possibly compounded with oxygen ions as a sulfate and/or sulfide ion.

Substrate 702 includes sulfur-scavenging species 705 that has a strong affinity for bonding with sulfur species 714, and therefore readily bonds with sulfur species 714 forming bound sulfur species 707. In this way, sulfur-scavenging species 705 “scavenges” inward-diffusing sulfur species 714 and prevents sulfur species 714 from reaching the zinc 706 at interface 708. Bound sulfur species 707 becomes locked within anodic oxide 704 and away from interface 708, and thus does not interfere with the adhesion capability of anodic oxide 704 to substrate 702.

Criteria for choosing sulfur-scavenging species 705 include how readily oxidized it is, its affinity for sulfur species 714, and its ionic mobility. Sulfur-scavenging species 705 should be more readily oxidized than aluminum such that, sulfur-scavenging species 705 is oxidized together with aluminum matrix 712 during anodizing. Table 1, below, lists a number of elements based on their calculated Gibbs free energy for oxide formation (−ΔG⁰ (kCal/mol O₂)).

TABLE 1 Oxide Formation Energies Element Oxide −ΔG⁰ (kCal/mol O₂) Yttrium (Y) Y₂O₃ 320 Calcium (Ca) CaO 320 Scandium (Sc) Sc₂O₃ 310 Europium (Eu) EuO 290 Gadolinium (Gd) Gd₂O₃ 287 Magnesium (Mg) MgO 286 Lanthanum (La) La₂O₃ 285 Lithium (Li) LiO₂ 285 Cerium (Ce) Ce₂O₃ 285 Strontium (Sr) SrO 280 Aluminum (Al) Al₂O₃ 277 Hafnium (Hf) HfO₂ 265 Zirconium (Zr) ZrO₂ 262 Erbium (Er) Er₂O₃ 260 Titanium (Ti) Ti₂O₃ 242 Silicon (Si) SiO₂ 217 Tantalum (Ta) Ti₂O₅ 198 Vanadium (V) VO 198 Manganese (Mn) MnO 184 Chromium (Cr) Cr₂O₃ 179 Niobium (Nb) NbO₂, NbO 176, 187 Zinc (Zn) ZnO 166 Rubidium (Rb) Rb₂O 158 Indium (In) In₂O₃ 157 Tin (Sn) SnO₂ 139 Tungsten (W) WO₃ 133 Iron (Fe) Fe₃O₄ 130 Germanium (Ge) GeO₂ 129 Molybdenum (Mo) MoO₂, MoO₃ 127, 106 Cobalt (Co) CoO 115 Nickel (Ni) NiO 114 Antimony (Sb) Sb₂O₃ 111 Bismuth (Bi) Bi₃O₄ 104 Copper (Cu) Cu₂O 80 Tellurium (Te) TeO₂ 77 Thallium (Tl) Tl₂O 69 Osmium (Os) OsO₂ 62 Selenium (Se) SeO₂ 54 Iridium (Ir) IrO₃ 43 Rhodium (Rh) Rh₂O₃ 42 Platinum (Pt) Pt₃O₄ 22 Silver (Ag) Ag₂O 14

Table 1 indicated that yttrium (Y), calcium (Ca), scandium (Sc), europium (Eu), gadolinium (Gd), magnesium (Mg), lanthanum (La), lithium (Li), cerium (Ce), and strontium (Sr) have more negative ΔG⁰ for oxide formation than aluminum (Al), and thus are more readily oxidized than aluminum (Al).

Another consideration for choosing viable candidates for the sulfur-scavenging species is an element's ability to form a stable compound with sulfur species 714. As described above, sulfur-scavenging species 705 can bind with sulfur species 714 to form a sulfate, a sulfide or other suitable stable bound sulfur species 707. Thus, one approximation as to an element's ability to bond with sulfur species 714 can be its enthalpy of sulfate formation.

Table 2, below, lists a number of elements based on enthalpy (−ΔH) of sulfate formation.

TABLE 2 Sulfate Formation Energies Element Sulfate −ΔH (kJ/mol SO₄) Barium (Ba) BaSO₄ 1473 Radium (Ra) RaSO₄ 1471 Strontium (Sr) SrSO₄ 1453 Lithium (Li) LiSO₄ 1437 Rubidium (Rb) RBSO₄ 1436 Calcium (Ca) CaSO₄ 1435 Sodium (Na) Na₂SO₄ 1387 Cerium (Ce) Ce₂(SO₄)₃ 1318 Magnesium (Mg) MgSO₄ 1285 Beryllium (Be) BeSO₄ 1250 Aluminum (Al) Al₂(SO₄)₃ 1147 Hafnium (Hf) Hf(SO4)₂ 1115 Zirconium (Zr) Zr(SO₄)₂ 1109 Manganese (Mn) MnSO₄ 1065 Zinc (Zn) ZnSO₄ 982 Chromium (Cr) Cr₂(SO₄)₃ 970 Iron (Fe) FeSO₄ 929 Palladium (Pd) PdSO₄ 920 Cadmium (Cd) CdSO₄ 935 Cobalt (Co) CoSO₄ 888 Iron (Fe) Fe₂(SO₄)₃ 861 Bismuth (Bi) Bi₂(SO₄)₃ 848 Nickel (Ni) NiSO₄ 872 Copper (Cu) Cu₂O 771 Gold (Ag) Ag₂SO₄ 716

Table 2 indicates that barium (Ba), radium (Ra), strontium (St), lithium (Li), rubidium (Rb), calcium (Ca), sodium (Na), cerium (Ce), magnesium (Mg), beryllium (Be), aluminum (Al), hafnium (Hf), zirconium (Zr), and manganese (Mn) each have very large negative (i.e., exothermic) values for ΔH for sulfate formation, and are therefore strong sulfate formers. Barium (Ba), radium (Ra), strontium (St), lithium (Li), rubidium (Rb), calcium (Ca), sodium (Na), cerium (Ce), magnesium (Mg), beryllium (Be) have more negative ΔH for sulfate formation than aluminum (Al), and therefore may be preferable in some embodiments.

In some preferred embodiments, the criterion for choosing the sulfur-scavenging species is based on both oxide formation and sulfate formation. Based on Tables 1 and 2, these elements include one or more of lithium (Li), magnesium (Mg), strontium (Sr), and calcium (Ca). It should be noted, however, that this does not necessarily include all possible suitable sulfur-scavenging species.

FIG. 8 shows an annotated periodic table illustrating some criteria for choosing a suitable sulfur-scavenging species, in accordance with some embodiments. Elements that have Gibbs free energy (ΔG⁰) for oxide formation greater than or equal to ΔG⁰ for aluminum oxide (Al₂O₃) formation can be eliminated. This leaves lithium (Li), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), scandium (Sc), yttrium (Y), lutetium (Lu), lawrencium (Lr), lanthanum (La), cerium (Ce), samarium (Sm), europium (Eu), gadolinium (Gd), actinium (Ac), and thorium (Th).

Some elements indicated in FIG. 8 can be further eliminated due to cost or other factors, such as toxicity or radioactivity (as indicated). For example, the cost of beryllium (Be), radium (Rd), scandium (Sc), yttrium (Y), lutetium (Lu), lawrencium (Lr), actinium (Ac), thorium (Th), samarium (Sm), europium (Eu), and gadolinium (Gd) may be too high for many practical purposes. Beryllium (Be), radium (Rd), yttrium (Y), lanthanum (La), actinium (Ac), cerium (Ce), and thorium (Th) may be toxic and/or radioactive. This leaves one or more of lithium (Li), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) as preferred sulfur-scavenging candidates, according to some embodiments.

An additional consideration for choosing a sulfur-scavenging species is its ionic mobility within the anodic oxide during anodizing. Those species that are lighter in weight, such as lithium, sodium and magnesium, have high mobility within an anodic oxide, thereby increasing the likelihood of reaching and binding with the counter-flowing sulfur species. Moreover, their high ionic mobility relative to aluminum ensures that under the applied electric field, they diffuse away from the metal oxide interface more rapidly than aluminum. Upon encountering counter-flowing sulfur species, they will form compounds away from the anodic oxide-substrate interface. They effectively present a counter-flowing barrier to the flow of such anions towards the interface.

It should be noted, however, that ion mobility might not be the only factor to consider. For example, Table 2 above shows that barium (Ba) forms a very strong bond with sulfate. However, the barium atom is much heavier than elements such as sodium and magnesium, generally making barium less effective for a given weight percent of lighter elements such as lithium, sodium and beryllium. Never the less, barium's higher sulfate energy formation may counterbalance its higher atomic weight.

Other considerations can include the solubility of the element within aluminum and the abundance of the element. Ideally, the elements in question should also present solubility within the aluminum matrix to the desired level, possibly also eliminating barium, calcium, cerium, or gadolinium in certain embodiments, For example, scandium is more soluble in face-centered cubic aluminum (0.04 atomic % at 500 Celsius) compared to yttrium (0.008 atomic % at 500 Celsius). However, this solubility condition may not be essential as being a suitable candidate, provided that the element does not have an adverse effect on the aluminum alloy's microstructure or on the anodic oxide cosmetics.

Low stability or low melting point may also be considered, which may rule out sodium, potassium, and rubidium. Elements such as scandium, europium, and hafnium are extreme scarce and may be further ruled out for this reason. In some embodiments where the above factors are considered, preferable sulfur-scavenging species include one or more of lithium, magnesium, calcium, strontium, and barium. In some embodiments, preferable sulfur-scavenging species include one or more of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium. In some embodiments, a combination of two or more of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium are used.

Other considerations include the cosmetic effects of adding the sulfur-scavenging species. As described above, some elements can noticeably discolor a resultant anodic oxide, with yellow discoloring particularly undesirable in certain applications. Lithium, magnesium, calcium, strontium, and barium have no significant yellowing effect on a resultant anodic oxide in amounts sufficient to increase delamination resistance, and can therefore be used without negatively affecting the cosmetic qualities of the anodic oxide.

The amount of sulfur-scavenging species can vary depending on the type of sulfur-scavenging species. For example, it may be desirable to add the sulfur-scavenging species to an amount sufficient to result in an anodic oxide having a predetermined determined delamination resistance, such as determined by techniques described in U.S. application Ser. No. 14/678,881, which is incorporated herein in its entirety. It should be noted, however, that the amount of sulfur-scavenging species added to the aluminum alloy to provide sufficient sulfur-scavenging capability is significantly greater than the micro-alloying amounts described above with respect to preventing enrichment of interface-weakening species. For example, it has been found that 2 weight % or more of lithium should be used in order to provide sufficient sulfur-scavenging and prevent delamination. However, high concentrations of alloying elements reduce the thermal conductivity of the alloy, which may be undesirable in some applications. In some embodiments, the sulfur-scavenging species is added to the substrate at a concentration ranging from about 0.5 weight % and about 3 weight %.

One alloying element that has proven to be a good sulfur-scavenging species is lithium. As described above, lithium added in a concentration of about 2 weight % proved to eliminate delamination. Whilst interfacial enrichment of zinc is shown to occur, the counter-migration of lithium through the anodic oxide is sufficient to limit the diffusion of sulfur species towards the interface, and to eliminate the formation of the interface-weakening species of zinc and sulfur—even when the anodizing is conducted under conditions which would result in a very weak interface in the absence of lithium (such as a conventional Type II anodizing process conducted in 200 g/L sulfuric acid at 1.5 A/dm² current density and 20 degrees Celsius). Furthermore, if the addition of lithium is used in combination with other approaches to minimized sulfate incorporation from the electrolyte, such as the mixed acid electrolyte compositions described in U.S. application Ser. No. 14/678,868 (which is incorporated herein in its entirety), a more robust solution can be achieved, and one or more of the necessary conditions may be relaxed (e.g., use less lithium, or higher current density, or lower anodizing bath temperature).

Magnesium, in particular, as a sulfur-scavenging species can be of interest since most commercially available 7000-series aluminum alloys already include magnesium and zinc as alloying elements. As described above, magnesium can be a key to the strengthening mechanism of many 7000-series alloys by virtue of its propensity to form precipitates such as MgZn₂, and specifically an η′ phase, within the aluminum matrix, within the aluminum matrix. FIG. 9 shows graph 900 indicating magnesium and zinc concentrations of different commercially available 7000 series aluminum alloys: 7005, 7108, 7003, 7029, 7075, 7050, 7030, 7046A, 7046, as well as custom aluminum alloy composition 904, which is based on optimal η′ precipitation strengthening, and custom aluminum alloy composition 906. The x-axis of graph 900 indicates weight % of zinc content and the y-axis indicates weight % of magnesium content within the aluminum alloys. Most of the commercial alloys include significant concentrations of other alloying elements which are not shown: 7029, 7030, 7046, 7050, and 7075, for example, all include copper at levels that would significantly discolor an anodic oxide.

Line 902 represents balanced zinc and magnesium compositions for providing MgZn₂ precipitates for enhancing the strength of the aluminum substrate. That is, line 902 represents stoichiometric amounts of zinc and magnesium to form MgZn₂ η′ precipitates. Alloy compositions below line 902 can be characterized as being zinc-rich, and alloy compositions above line 902 can be characterized as being magnesium-rich. Excess zinc or magnesium in zinc-rich or magnesium-rich alloy compositions will reside in the aluminum matrix of the aluminum alloy, reducing the thermal or electrical conductivity of the alloy. Thus, for some applications, and notably for electronics enclosures, which play a role in dissipating heat, it is preferable to avoid this by choosing aluminum alloys having a generally balanced magnesium and zinc composition, such as custom aluminum alloy composition 904. In addition to the Mg:Zn ratio defined by the precipitate stoichiometry, an optimal level (volume fraction) of precipitate strengthening has informed the selection of exact composition target 904, allowing for a given homogenization, quenching, extrusion and artificial ageing process to achieve a target strength, such as 340 MPa.

Since magnesium can also act as an effective sulfur-scavenging species, an excess of magnesium over the balanced composition of custom alloy 904 can be made, such as indicated magnesium-rich custom alloy 906. That is, a magnesium-rich custom alloy 906 may be beneficial in providing the added benefit of sulfur-scavenging and thereby improving adhesion of an anodic oxide. The amount of excess magnesium relative to zinc to provide such sulfur-scavenging benefit can be significant. Namely the atomic concentration of magnesium should be at least equal to half the atomic concentration of zinc, and preferably equal to or greater than the atomic concentration of zinc, placing it in excess by a factor of two. The excess of magnesium in this illustrative example is not intended to change the strengthening precipitate from MgZn₂ (though it may do so, as detailed in the next paragraph), and as such, it does not significantly contribute strength to the alloy. Nor does the excess of magnesium have a significant effect on the level of zinc accumulation at the interface, even though it can reduce or eliminate the level of free zinc in the matrix. This is because the interfacial accumulation of zinc during anodizing occurs whether the zinc is in the matrix or bound in a precipitate phase. In sufficient excess, however, the excess magnesium does prevent delamination of the anodic oxide through this sulfur-scavenging mechanism. It may reduce the thermal and electrical conductivity of the alloy somewhat, but may be beneficial in terms of corrosion resistance.

Although, the excess of magnesium in the previous illustrative example is not intended to significantly contribute strength to an alloy where the strengthening phase was assumed to be MgZn₂, alternative strengthening phases may be formed in some cases, and their roles must then be considered in determining precise alloying concentrations for optimal strengthening under any given thermo-mechanical processing route, and in turn for determining an appropriate excess of magnesium. This may allow equivalent strength to alloy 904 with lower zinc concentrations. Whatever the precise alloy composition selected for optimal strengthening, an excess of magnesium can be preferred. A wide region of interest for such candidate alloys exists, in the region outlined 908. Note that, in addition to custom compositions 904/906, region 908 encompasses lower zinc compositions, including those above and to the left of 904/906 on graph 900. A further benefit of the excess of magnesium will be discussed later.

It should be noted that the commercially available aluminum alloys shown in FIG. 9 include significant amounts of elements other than zinc and magnesium, such as copper, iron, silicon and manganese, which can have undesirable cosmetic effects. As described above, the presence of copper, manganese, and certain other elements in more than micro-alloying amounts can yellow a resultant anodic oxide to unacceptably high b* value levels. As described above, in some applications the b* value of the anodized part, as viewed from an exposed surface of the anodic oxide (e.g., surface 701), should be less than 2, in some cases less than 1, or even less than 0.5. Thus, in some embodiments, custom alloy 904 and magnesium-rich custom alloy 906, have very strictly controlled maxima for all other elements: e.g., 0.01 weight % for magnesium, 0.01 weight % for chromium, 0.01 weight % for zirconium, 0.02 weight % for copper, 0.02 weight % for titanium, 0.05 weight % for silicon, and a maximum of 0.01 weight % for any other non-specified element, to a total maximum of 0.1 weight % for non-specified others. Other elements may, however, be specifically added as micro-alloying elements in accordance with the approach described earlier in this paper.

It has been found that zinc and magnesium do not yellow a resultant anodic oxide. In fact, magnesium and/or zinc may tend to provide a bluish high to the anodic oxide, which in certain applications is more desirable than a yellow hue. Thus, magnesium-rich custom alloy 906 can provide sulfur-scavenging capability as well as desired cosmetic (color) quality to an anodized part.

FIG. 10 shows flowchart 1000, illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate using a sulfur-scavenging strategy. The aluminum alloy substrate can include zinc and magnesium alloys that give the substrate high tensile strength, with a balanced proportion of magnesium and zinc (e.g., atomic % zinc=2 times atomic % magnesium to yield MgZn₂ η′ precipitates). At 1002, a sulfur-scavenging species is incorporated into an aluminum alloy substrate. In some embodiments, the sulfur-scavenging species is added at a concentration ranging from about 0.5 weight % and about 3 weight %. In a particular embodiment, the sulfur-scavenging species is an excess of magnesium (i.e., a significant addition of magnesium, over and above the balanced level of half that of the atomic % zinc).

In some embodiments, the sulfur-scavenging species has a Gibbs free energy of oxide formation lower than that of aluminum. In some embodiments, the sulfur-scavenging species is additionally a strong sulfate former, i.e., has a large negative enthalpy for sulfate formation—in some embodiments, more negative than that for aluminum sulfate formation. In some embodiments, the sulfur-scavenging species includes one or more of lithium, magnesium, calcium, strontium, and barium. In particular embodiments, two or more types of sulfur-scavenging species are used.

At 1004, the aluminum alloy substrate is anodized, using, for example, an anodic solution comprising sulfuric acid (e.g., type II anodizing process). During anodizing, the sulfur-scavenging species binds with sulfur species originating from the anodic solution. For example, lithium and/or magnesium and bind with sulfate ions to form lithium sulfate and/or magnesium sulfate. In this way, the sulfur-scavenging species “scavenges” the sulfur species and prevents the sulfur species from binding with zinc to form a zinc-sulfur species, which is an interface-weakening species, at an interface between the substrate and a resultant anodic oxide.

A further possible benefit of engineering an excess of magnesium, and the correspondingly reduced level of free zinc in the matrix is that the differential growth rates of grains of different orientations (by virtue of zinc making the matrix more anodic in {111} orientations) may be eliminated. Thus, aluminum-zinc-magnesium alloys with an excess of magnesium are preferred for avoiding the afore-mentioned grain-orientation related cosmetic defects. In a zinc-rich alloy, or an alloy with a balanced ratio (such as Zn:2×Mg where MgZn₂ precipitates are expected), anodic oxide forms on grains of {111} surface orientation approximately 10% faster than on grains on other surface orientations (under typical Type II anodizing conditions at 1.5 A/dm²). When a substantial excess of magnesium is employed to eliminate free zinc in the matrix, this discrepancy is reduced to about 1-3% and the visual defect is eliminated. That is, the magnesium can be added in excess over a balanced ratio for magnesium-zinc precipitate formation so as to eliminate or reduce a concentration of non-precipitated zinc in the aluminum alloy substrate in a T6 or T7 temper. This reduces a discrepancy between growth rates of different portions of the anodic oxide on grains of distinct surface orientations, resulting in the anodic oxide having a thickness uniformity of within 5% among grains of {111} surface orientation and other surface orientations.

FIG. 12 shows how the addition of copper in micro-alloying amounts and lithium as a sulfur-scavenging species can improve adhesive of an anodic film to a substrate. In particular, FIG. 12 shows scanning electron microscope (SEM) images of three anodized substrate samples (1200, 1202, 1204) after performing a 5-by-5 array of 10 kg Vickers indentations spaced 350 micrometers apart, using an interfacial adhesion testing method as disclosed in U.S. patent application Ser. No. 14/678,881, which is disclosed herein in its entirety. In all samples 1202, 1204, and 1204, the anodic oxides are of 14 micrometer thickness, and were formed using 1.5 A/dm² current density in 200 g/L sulfuric acid solution.

Sample 1200 is an aluminum alloy substrate having 5.5 weight % of zinc and 1.0 weight % of magnesium (corresponding to a balanced zinc and magnesium aluminum alloy) without added micro-alloying element or sulfur-scavenging species. The SEM image of sample 1200 shows evidence of significant anodic oxide detachment due to interface weakening by interfacial enrichment of zinc, and its interaction with sulfur from the anodizing electrolyte. In particular, the back-scattered compositional scanning electron microscope image of sample 1200 shows a number of light areas corresponding to the bare aluminum substrate—where the applied load has detached the anodic oxide. Some manufacturing requirements require samples having less than 10 detachment regions to be acceptable.

Sample 1202 is an aluminum alloy substrate having 5.5 weight % of zinc, 1.0 weight % of magnesium, and 0.05 weight % copper as a micro-alloying element. As shown, the indentation test shows there are only four very much smaller light areas corresponding to interfacial adhesion failure. Thus, the addition of just a small amount of copper is sufficient to overcome the weak interfacial adhesion caused by interfacial enrichment of zinc and zinc-sulfur species. Sample 1204 is an aluminum alloy substrate having 5.5 weight % of zinc, 1.0 weight % of magnesium, and 1.75 weight % lithium as a sulfur-scavenging element. The SEM image of sample 1204 shows substantially no bright spots, thereby indicating the addition of a sulfur-scavenging element (e.g., lithium) also overcomes the delamination problem.

Combinations and Other Embodiments

In some cases, using a combination of a micro-alloying element and a sulfur-scavenging species has been found to provide a combined benefit. For example, adding one or more micro-alloying elements and adding one or more sulfur-scavenging species to an aluminum alloy composition can result in an anodic oxide having an even higher resistance to delamination than the micro-alloying element or sulfur-scavenging species individually. For example, copper micro-alloying (which minimize zinc enrichment at the interface) may be used in combination with lithium and/or magnesium for their sulfur-scavenging ability. Alternatively, one or more sulfur-scavenging species can be added in lower amounts than would be used to prevent delamination alone, and one or more micro-alloying elements can be added to make up for the deficiency in sulfur-scavenging species, resulting in an anodic film that is resistant to delamination. This strategy can be used to add lesser amounts of elements that can cause yellow discoloration, such as copper, iron, and/or silver.

FIG. 11 shows flowchart 1100, illustrating a process of increasing an adhesion strength of an anodic oxide to a high-strength substrate using a combination of sulfur-scavenging species and micro-alloying element. At 1102, relative amounts of one or more sulfur-scavenging species and one or more micro-alloying elements required to achieve a pre-determined delamination resistance of an anodic oxide on a high aluminum strength substrate is determined. The pre-determined delamination resistance can be a threshold value determined using, for example, the delamination-resistance methods described in U.S. application Ser. No. 14/678,881, which is incorporated herein in its entirety. Additionally or alternatively, the relative amounts of a sulfur-scavenging species and a micro-alloying element can be determined by a pre-determined discoloration of the anodic oxide. For example, the pre-determined discoloration may be not be allowed to exceed a particular b* value (e.g., b*>1 or b*>0.5).

At 1104, the one or more sulfur-scavenging species and the one or more micro-alloying element are incorporated into an aluminum alloy substrate. At 1106, the aluminum alloy substrate is anodized such that during anodizing, the sulfur-scavenging species binds with sulfur species, and the micro-alloying element prevents at least some of the zinc from enriching at the interface between the substrate and anodic oxides. The combination of the sulfur-scavenging species and the micro-alloying element increases the adhesion strength of the anodic oxide and/or reduces the discoloration of the anodic oxide compared to using a sulfur-scavenging species or a micro-alloying element individually.

In some cases, the addition of one or more sulfur-scavenging species and/or one or more micro-alloying elements can reduce the amount of other alloying elements required to provide the high-tensile strength to the aluminum alloy. For example, adding 0.05 weight % of copper micro-alloying element has been found to reduce the amount of zinc required for optimum strengthening by a corresponding 0.05 weight %, with the resultant aluminum alloy substrate having substantially the same mechanical properties (e.g., yield strength of 340-350 MPa, and hardness of 125 HV), as the alloy having the full concentration of zinc. In a particular embodiment, this reduces the zinc composition to 5.45 weight % compared to a nominal 5.5 weight % for the custom alloy 904 of FIG. 9. This can be some benefit since aluminum alloys having higher zinc composition can present corrosion problems while lower zinc compositions (without the micro-alloying element) can detrimentally affect the strength of the alloy substrate. Once the amount of zinc is fixed, a corresponding amount of magnesium to provide a balanced alloy can be determined. In some embodiments, the magnesium content is substantially increased over this balanced amount to provide the sulfur-scavenging benefits described above or to minimize free zinc in the matrix, so as to reduce grain-orientation related defects. In this way, customized anodized alloys having prescribed tensile strength, anodic oxide delamination resistance, and/or color can be designed.

It should be noted that embodiments presented herein can be used in combination with one or more embodiments described in related U.S. application Ser. Nos. 14/474,021, 14/593,845, 14/678,881, and 14/678,868, which are incorporated herein in their entireties. For example, the embodiments described herein may be used in combination with a post-anodizing heat treatment to diffuse zinc away from the interface, either for greater robustness, or to allow for shorter or lower temperature heat treatments to achieve the same effect.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 

What is claimed is:
 1. A part, comprising: an aluminum alloy substrate including zinc as an alloying element; and an anodic oxide coating formed on the aluminum alloy substrate, wherein the zinc is enriched at an interface between the anodic oxide coating and the aluminum alloy substrate, wherein the aluminum alloy substrate includes a sulfur-scavenging species at a sufficient concentration to bind with sulfur species within the anodic oxide coating preventing at least some of the enriched zinc from forming a zinc-sulfur compound at the interface, the zinc-sulfur compound associated with reducing an adhesion strength between the anodic oxide coating and the aluminum alloy substrate.
 2. The part of claim 1, wherein the aluminum alloy substrate includes additional elements other than zinc, magnesium, and the sulfur-scavenging species, wherein the additional elements comprise: chromium at no more than 0.01 weight % concentration, copper at no more than 0.01 weight % concentration, manganese at no more than 0.01 weight % concentration, zirconium at no more than 0.01 weight % concentration, titanium at no more than 0.02 weight % concentration, silicon at no more than 0.05 weight % concentration, iron at no more than 0.08 weight % concentration, and any other element at no more than 0.01 weight % concentration, to a total maximum of 0.1 weight % concentration of the additional elements.
 3. The part of claim 1, wherein the anodic oxide coating has a thickness of at least about 10 micrometers and is characterized as having b* color space parameter value between −1 and 1, optionally between −0.5 and 0.5, as defined by CIE Standard Illuminant D65 white spot standard.
 4. The part of claim 1, wherein an adhesion strength of the anodic oxide coating as measured by 5-by-5 array 10 kg Vickers indentations spaced 350 micrometers apart and as viewed by scanning electron microscope imaging is less than 10 detached regions of the anodic oxide coating.
 5. The part of claim 1, wherein the aluminum alloy substrate has a yield strength of at least 330 MPa in a T6 temper.
 6. The part of claim 1, wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium.
 7. The part of claim 1, wherein a concentration of the sulfur-scavenging species ranges from about 0.5 weight % and about 3 weight %.
 8. The part of claim 1, wherein magnesium is added in excess over a balanced ratio for magnesium-zinc precipitate formation so as to eliminate or reduce a concentration of non-precipitated zinc in the aluminum alloy substrate in a T6 or T7 temper, thereby reducing a discrepancy between growth rates of different portions of the anodic oxide coating on grains of distinct surface orientations, resulting in the anodic oxide coating having a thickness uniformity of within 5% among grains of {111} surface orientation and other surface orientations.
 9. The part of claim 8, wherein a concentration of the magnesium is in excess of a stoichiometric amount required to combine with the zinc to form η-MgZn₂ precipitates, wherein at least some of the excess magnesium binds with the sulfur species within the anodic oxide coating preventing at least some of the zinc from forming a zinc-sulfur compound at an interface between the anodic oxide coating and the aluminum alloy substrate.
 10. The part of claim 1, wherein the aluminum alloy substrate comprises magnesium, wherein an atomic concentration of the magnesium is at least half an atomic concentration of the zinc.
 11. The part of claim 1, wherein the part is an enclosure for an electronic device.
 12. The part of claim 1, wherein the aluminum alloy substrate is comprised of a 7000-series aluminum alloy.
 13. A method of anodizing an aluminum alloy substrate comprising zinc, the method comprising: anodizing the aluminum alloy substrate in a sulfuric acid-based solution, wherein a sulfur species from the sulfuric acid-based solution becomes incorporated within a resultant anodic oxide coating, wherein some of the zinc becomes enriched at an interface between the anodic oxide coating and aluminum alloy substrate during the anodizing, wherein the aluminum alloy substrate includes a sulfur-scavenging species that binds with the sulfur species preventing at least some of the enriched zinc from forming a zinc-sulfur compound at the interface, the zinc-sulfur compound associated with reducing an adhesion strength between the anodic oxide coating and the aluminum alloy substrate.
 14. The method of claim 13, wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium.
 15. The method of claim 14, wherein a concentration of the sulfur-scavenging species ranges from about 0.5 weight % and about 3 weight %.
 16. The method of claim 13, wherein a concentration of the zinc within the aluminum alloy substrate is at least 4 weight %.
 17. The method of claim 13, wherein the aluminum alloy substrate includes magnesium as another alloying element, wherein an atomic concentration of the magnesium is at least half an atomic concentration of the zinc.
 18. An enclosure for an electronic device, the enclosure comprising: an aluminum alloy substrate including zinc and magnesium; and an anodic oxide coating formed on the aluminum alloy substrate, the anodic oxide coating including a sulfur species incorporated therein, wherein some of the sulfur species is bonded with a sulfur-scavenging species that prevents the sulfur species from binding with the zinc.
 19. The enclosure of claim 18, wherein the aluminum alloy substrate with the anodic oxide coating is characterized as having a CIELAB b* color space parameter value between −1 and
 1. 20. The enclosure of claim 18, wherein the sulfur-scavenging species is selected from the group consisting of lithium, magnesium, calcium, strontium, barium, scandium, and yttrium. 